Method for determining an adjustment amount to an input chroma

ABSTRACT

A method is disclosed for determining an adjustment amount to be made to an input chroma, C in , to squeeze the input chroma toward a region of preferred chroma, C pref . This method involving first defining a change in chroma as: ΔC=C in −C pref  and defining a chroma weight as: C weight =Gaussian(C pref ,C sigma ); defining a luminance weight as: L weight =Gaussian(L pref ,L sigma ); defining a hue weight as: H weight =Gaussian(H pref ,H sigma );. Then, an amount of chroma adjustment is: C Adjust =ΔC*(H weight *C weight *L weight ). An output chroma is generated by applying chroma adjustment to chroma input: C out =C in −C Adjust .

RELATED APPLICATIONS

Cross reference is made to the following applications filed concurrentlyherewith: Attorney Docket Number D/A2068 entitled “A METHOD FORDETERMINING A HUE ADJUSTMENT TO AN INPUT HUE” by Karen M. Braun; andAttorney Docket Number D/A20680 entitled “A METHOD FOR SQUEEZING ANINPUT HUE TOWARD A REGION OF PREFERRED HUE” by Karen M. Braun.

FIELD OF THE INVENTION

The present invention generally relates to methods for converting colorvalues and, more particularly, to methods for adjusting input colors inthe direction of a specified output color to improve image appearance.

BACKGROUND OF THE INVENTION

Film companies are known to change certain colors to, for instance, showskin more tanned or the sky a different shade of blue. This isrelatively easy when one has control over most every aspect of the imagecapturing and image production process. However, when the input imagehas been captured and rendered in an unknown way, improving certaincolors therein becomes more complicated. This often involves segmentingcolors believed to make up a certain color then adjusting these toward apreferred color point. This is computationally intensive, subject tofailure, and often results in non-smooth transitions between differentcolor regions such as skin color and non-skin color areas.

Another approach is to specify both original and desired color and thenapply a vector from original to desired color with the effect of thevector smoothly decaying for nearby colors in color space. One problemwith this approach is non-monotonic behavior leading to color reversals.Other techniques involve a weighting function for transforming a colorto a new color and smoothly adjusting surrounding colors withoutinducing any tone reversals using a Gaussian weight on the differencebetween the input and the desired output. This method also uses aconditional function to insure monotonicity of the input to outputrelationship. Another technique transforms an input color to a desiredoutput color and colors surrounding the input color are appropriatelywarped to give smooth and monotonic output using a concept of colorgravity wells toward which other colors are adjusted. However, thistechnique is specific to neutrals and pure primaries and secondaries.

What is needed in this art is a method for adjusting input colors in thedirection of a specified output color to improve image appearance thatacts on colors in color space rather than adjusting the colors ofcertain pixels spatially to robustly render critical colors which areknown a priori.

BRIEF SUMMARY

A method is disclosed for determining an adjustment amount to be made toan input chroma, C_(in), to squeeze the input chroma toward a region ofpreferred chroma, C_(pref). This method involving first defining achange in chroma as: ΔC=C_(in)−C_(pref) and defining a chroma weightsuch that the weight approaches 1 at the peak of the Gaussian as:C_(weight)=Gaussian(C_(pref),C_(sigma)). Then, an amount of chromaadjustment is then: C_(Adjust)=ΔC*C_(weight). An output chroma isgenerated by applying chroma adjustment to chroma input:C_(out)=C_(in)−C_(Adjust). Similarly, disclosed is method fordetermining an adjustment amount to be made to an input lightness.

The present invention offers a plurality of advantages. For example, theinput can also be squeezed toward a point in either RGB, a*b*, or u′v′space. Multiple hue centers can be use to sequentially squeeze the inputtoward a region of a preferred skin, sky, or grass. In the case ofmultiple squeezes, finite non-overlapping regions of support maintainthe shifts from previous squeezes. The inputs can be pre-specified in acolor management system or specified and provided dynamically. Whensqueezing is applied in a non-uniform way, a left side of a weightingcurve and a right side of another are used to find regions of rapiddecrease in quality for positive ΔH*_(ab) renditions of certain tones.

DESCRIPTION OF THE SPECIFICATION

One method is disclosed for determining a hue adjustment to an inputhue, H_(in), to squeeze the input hue toward a region of preferred hue,H_(pref). The method involves obtaining a change in hue as:ΔH=H_(in)−H_(pref) and a hue weight such that the weight approaches 1 atthe peak of the Gaussian as: H_(weight)=Gaussian(H_(pref),H_(sigma)). Anamount of hue adjustment is calculated by: H_(Adjust)=ΔH*H_(weight).Then, an output hue is generated by applying the adjustment such that:H_(out)=H_(in)−H_(Adjust).

Change in hue, ΔH, is the difference between the input hue, H_(in) isthe hue of a given pixel in the image (or node in the table) and thepreferred hue, H_(pref).ΔH=H _(in) −H _(pref)H _(weight)=Gaussian(H _(pref) ,H _(sigma))H _(Adjust) =ΔH*H _(weight)H _(out) =H _(in) −H _(Adjust)

The weight approaches 1 at the peak of the Gaussian. When H_(in) isclose to the preferred hue, H_(pref), the ΔH causes the hue adjustment(H_(Adjust)) to be small so the output hue is very similar to the inputhue (not rotated completely to the preferred hue). Hues very close to,or very far from, the preferred hue, get the least amount of shift.

Also disclosed is a method for squeezing an input hue, H_(in), toward aregion of preferred hue, H_(pref), having a preferred chroma, C_(pref),and luminance, L_(pref), to restrict the rotation effect to a point inLCH space rather than an entire hue slice. This method involving firstdefining a chroma weight as: C_(weight)=Gaussian(C_(pref),C_(sigma)).Then, defining a luminance weight as:L_(weight)=Gaussian(L_(pref),L_(sigma)). Defining a hue weight thatapproaches 1 at the peak of a Gaussian as:H_(weight)=Gaussian(H_(pref),H_(sigma)). Defining an amount of hueadjustment as: H_(adjust)=ΔH*(H_(weight)*C_(weight)*L_(weight)). Then,generating an output hue by applying hue adjustment to hue input suchthat: H_(out)=H_(in)−H_(Adjust).

To restrict the rotation effect to be toward a point in LCH space,rather than toward an entire hue slice:C _(weight)=Gaussian(C _(pref) ,C _(sigma))L _(weight)=Gaussian(L _(pref) ,L _(sigma))H _(weight)=Gaussian(H _(pref) ,H _(sigma))H _(adjust) =ΔH*(H _(weight) *C _(weight) *L _(weight))H _(out) =−H _(adjust)

Alternately, the weighting function was altered to be a Gaussian wasconvolved with a Rect function, which allows independent control overhow much to adjust colors and how far away from H_(pref) to change thecolors.

Preferably, the weighting function is replaced by the addition of twoGaussians, which allows additional flexibility over how colors are movedtoward the preferred point by allowing additional parameters (K, M) forthe adjustment.${weight\_ tmp} = {{\mathbb{e}}^{\frac{- {({{H\_ in} - M})}^{2}}{2*{H\_ sigma}^{2}}} + {\mathbb{e}}^{\frac{- {({{H\_ in} + M})}^{2}}{2*{H\_ sigma}^{2}}}}$ H _(weight) =K*weight_tmp/max(weight_tmp)The constant K in the second equation is adjusted such that nocrossovers occur in hue. This parameter is adjusted such that therelationship between H_(out) and H_(in) is monotonically increasing.

Also disclosed is a method for determining an adjustment amount to bemade to an input chroma, C_(in), to squeeze the input chroma toward aregion of preferred chroma, C_(pref). This method involving firstdefining a change in chroma as: ΔC=C_(in)−C_(pref) and defining a chromaweight as: C_(weight)=Gaussian(C_(pref),C_(sigma)). Then, an amount ofchroma adjustment is then:C_(Adjust)=ΔC*(H_(weight)*C_(weight)*L_(weight)). An output chroma isgenerated by applying chroma adjustment to chroma input:C _(out) =C _(in) −C _(Adjust).

The change in chroma, ΔC, is the difference between the chroma ofinterest, C_(in)and the preferred chroma, C_(pref).ΔC=C _(in) −C _(pref)C _(weight)=Gaussian(C _(pref) ,C _(sigma))L _(weight)=Gaussian(L _(pref) ,L _(sigma))H _(weight)=Gaussian(H _(pref) ,H _(sigma))C _(Adjust) =ΔC*(H _(weight) C _(weight) *L _(weight))C _(out) =C _(in) −C _(Adjust)

Although the current description is directed toward threeone-dimensional Gaussian functions, it is intended to also be directedtoward a single three-dimensional Gaussian.

The present method is also extensible to squeezing colors toward a pointin RGB space, a*b* (as opposed to h°/C*) space, u′v′ space, etc.

Because of the sequential nature of the present method, colors aresqueezed toward a point and not shifted directly to that point. Colorsnear the preferred point don't change much as the hue adjustmentapproaches 0, and this prevents hues from crossing the preferred pointor hue reversals.

The present invention is additionally applicable for multiple huecenters, for example, to squeeze toward preferred skin, sky, and grass.This is done sequentially, but as each subsequent squeeze occurs, theformer preferred points also be inadvertently shifted. Therefore, in thecase of multiple squeezes, finite non-overlapping regions of support aredefined making certain to produce smooth transitions between theseregions. The squeezing is alternately applied in a non-uniform way, forexample, to account for a rapid decrease in quality for positiveΔH*_(ab) (yellow) renditions of skin tones. This only involves using theleft side of one weighting curve and the right side of another; this isaccomplished using two different values of H_(sigma).

The present method can be applied in a device or in an abstract profile.The preferred colors can be pre-specified in the color management systemor dynamically specified by the user.

Further advantages include no need for segmentation or other knowledgeof input image; no need to specify original color or color vector;monotonicity (no crossovers), smoothness, and flexibility (e.g., thesize of the region of surrounding colors this is affected by the“squeezing” can be varied).

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications variations, improvements, and substantial equivalents.

1. A method for determining an adjustment amount to be made to an input chroma, C_(in), to squeeze the input chroma toward a region of preferred chroma, C_(pref), comprising: a) defining a change in chroma as: ΔC=C_(in)−C_(pref); b) defining a chroma weighting function; c) defining an amount of chroma adjustment as: C_(Adjust)=ΔC*(H_(weight)*C_(weight)*L_(weight)); and d) generating an output chroma by applying chroma adjustment to chroma input as follows: C_(out)=C_(in)−C_(Adjust).
 2. A method, as defined in claim 1, comprising a Gaussian weighting function: C_(weight)=Gaussian(C_(pref),C_(sigma)).
 3. A method, as defined in claim 1, wherein the lightness weighting function is defined by the Gaussian function: L_(weight)=Gaussian(L_(pref),L_(sigma)).
 4. A method, as defined in claim 1, wherein the three one-dimensional weighting functions are replaced by a three-dimensional weighting function.
 5. A method, as defined in claim 1, wherein the input is squeezed toward a point in a predetermined colorspace e.g., RGB, a*b*, or u′v′ space.
 6. A method, as defined in claim 1, wherein, in the case of multiple squeezes, defining finite non-overlapping regions of support.
 7. A method, as defined in claim 1, wherein inputs are pre-specified in a color management system.
 8. A method, as defined in claim 1, wherein the inputs are dynamically specified by the user.
 9. A method, as defined in claim 1, wherein the squeezing is applied in a non-uniform way by using one weighting function at input chroma values less than the preferred chroma and another weighting function at input chroma values greater than the preferred chroma.
 10. A method, as defined in claim 1, wherein L_(weight)is set to 1 causing the squeezing to be toward a plane of preferred hue and chroma.
 11. A method, as defined in claim 1, wherein a single 3-dimensional weighting function is used instead of three 1-dimensional functions. 